A boundary microphone is also called a surface-mount microphone because of its flat entire shape and is placed and used, for example, on a table, a floor surface or the like in a conference room.
For the boundary microphone, an omnidirectional microphone unit is used in many cases. There can be a case where narrow directivity is required to excellently collect sound even at a distance relatively separate from a sound source.
In such a case, on the boundary microphone, a narrow directional microphone in which an acoustic tube is attached to a unidirectional microphone unit is mounted. In this type of narrow directional microphone having an acoustic tube, a thin and long slit-shaped opening (side opening for sound wave) is provided on the tube wall of the acoustic tube along the axial line of the acoustic tube, an acoustic resistance material is attached to the side opening for sound wave, and a sound wave is taken in the acoustic tube.
Now, on the boundary microphone, liquid (for example, drinking water or the like spilt due to a glass falling) is more often splashed than on a stand-type microphone, a handheld microphone or the like.
In the narrow directional microphone having an acoustic tube, a non-woven cloth, a mesh body or the like is typically used for the acoustic resistance material attached to the side opening for sound wave of the acoustic tube. When the liquid penetrates this, causing wetness, there arises inconvenience of change in directivity characteristics or the similar situation.
Regarding this point, for example, Patent Literature 1 (Japanese Patent Application Publication No. 2006-101314) and Patent Literature 2 (Japanese Patent Application Publication No. 2008-288933) propose that a joint part between a base plate and a cover plate (made of a punching metal) of a boundary microphone and the cover plate undergo water repellent treatment.
However, when a large amount of liquid is splashed thereon, there is a concern that a part thereof enters the inside of the boundary microphone through the holes of the punching metal which is the cover plate and penetrates the acoustic resistance material of the acoustic tube. Hence, in the boundary microphone using the narrow directional microphone having an acoustic tube, there are needed more secure waterproof (water resistant) countermeasures for the acoustic tube.
Accordingly, an object of the present invention is to protect an acoustic tube in particular from liquid entering from the outside, in a narrow directional boundary microphone using a narrow directional microphone having the acoustic tube.